The take-home impression I got from reading through those two Cesar wars is that the legions were almost as dependent on local grain available to steal (ready to take in granary, or just ready to harvest, doesn't really matter) as trains are dependent on rails. I assume that except for bumping into an adequate rival in the east, they just gobbled up the entire "wheatosphere", quickly running out of steam (and, with a few notable exceptions, out of motivation) whatever they ran into hunter/gatherer economies.
Central and Western Europe have been farmed for thousands of years by that point. Farming in Southern France/Iberia was already well established around 7000 years BP (before present). By the time of Romans, the hunter-gatherer's lifestyle was wholly displaced from the area, with only minuscule fraction of resident population engaging in it, at best.
After all the grain has been stolen, you are in for a rough year where hunter/gathering might reappear + cannibalism. Imagine it less as a "civilization appears" moment, and more a "metal locusts from outer space eat previous civilization and leaf behind large slave farms after years of famine.
That's what I meant: I take it as a given that if they were successfully invaded, they must have left the hunter/gatherer state behind, likely by a considerable margin.
